There is increasing pressure on young consumers to practice sustainable consumption. With young women being key agents in fashion consumption, switching their purchasing to secondhand clothing over new is instrumental to reducing textile waste. This study applied the Theory of Planned Behaviour and Prototype Willingness Model to identify key drivers informing young women’s secondhand clothing purchasing decisions. Young Australian women (N = 48) completed qualitative surveys assessing their underlying attitudinal, normative, and control beliefs and perceived images of typical secondhand clothing shoppers. Thematic analysis indicated the main benefits of secondhand clothing purchasing to be the environmental impact and cost savings, with drawbacks being quality issues, reduced shopping experience, and greater effort required. Clothing diversity was both positive (‘unique finds’) and had a downside (limited sizes). Approvers of secondhand purchasing were mainly friends and family, with older relatives being less supportive. Key barriers were increased prices for quality items and the time required to locate them. Images of typical secondhand clothes shoppers were generally positive (‘cool’, ‘thrifty’, ‘unique’, ‘eco-friendly’), while ‘materialistic’, ‘upper-class’, and ‘ignorant’ but also ‘trendy’ indicated mixed perceptions about those who did not. Crucial in our findings was clarifying the intersections and contextual context of participants’ responses. Identifying the nuances in the underlying beliefs driving young women’s fashion choices assists in theory-informed strategies to encourage sustainable consumption of clothing.